14 Birds of Colorado 
Habits.—The Parasitic Jaeger, or as it is often called, 
Richardson’s Skua, is remarkable for presenting, irre- 
spective of age or sex, two very distinct phases of plumage 
—a light andadark. Birds of both phases pair with one 
another indiscriminately, where they meet, and the 
young are sometimes intermediate. This bird is parasitic 
in its mode of life; it seldom catches fishes for itself, 
but is constantly chasing smaller Gulls and Terns, and 
compelling them to disgorge their prey. So active 
is the Skua that it usually manages to catch the fish 
as it drops from the bill of the Gull, before it reaches 
the surface of the water. 
It is a purely marine form, but seldom seen inland, 
though it nests in the far north, on moorlands and 
swamps, using a slight depression in the ground scantily 
lined with grasses. The eggs, usually two in number, 
are olive-brown, marked heavily with chocolate. 
Family LARIDZ. 
Bill without cere, middle tail-feathers never elongated. 
Genus RISSA. 
In most respects resembling Larus, but the hind toe rudimentary 
or absent, very rarely with a claw ; tail square ; tarsus short, less than 
the middle toe without claw ; toes fully webbed. 
Two species only of circumpolar range. 
Kittiwake. Rissa tridactyla. 
A.O.U. Checklist no 40—Colorado Records—Ridgway 79, p. 232; 
Cooke 97, p. 50; Henderson 09, p. 225. 
Description.—Adult—Head, neck, tail and under-parts pure white, 
back and wings pearl-grey ; outer web of the first primary and the 
last three inches of the tip of both first and second primaries black ; 
the third to fifth black subterminally with an increasing white tip ; 
hind toe very small—a minute knob without claw ; bill yellowish, feet 
black. Length 17; wing 12; tail 4-5; culmen 1-40; tarsus 1-3. 
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